1. Field of the Invention
The present invention relates to a method for repairing a pattern defect on a photo mask, the photo mask repaired by the repairing method, and a manufacturing method of semiconductor devices employing the repaired photo mask. The photo mask may include a phase shift mask.
2. Description of the Related Art
In the manufacturing method of the semiconductor devices such as large scale integrations (LSls), very large scale integrations (VLSIs), ultra large scale integrations (ULSIs), and giga scale integrations (GSIs), a set of photo masks or reticles is required for photolithography steps. Each of the photo masks is composed of predetermined patterns made of a light-shielding layer or phase shift layer arranged on a transparent mask substrate, such as a quartz substrate. During the fabrication process of the photo masks, miscellaneous microscopic defects 6a, 6b, 6c may be created on the mask substrate as shown in FIGS. 1A, 1B and 1C, for example. In each of FIGS. 1A to 1C, three lines 3a, 3b, and 3c made of the light-shielding layer are shown, sandwiching spaces 4 and 4 between them. In FIG. 1A, a defect 6a protruding from the left edge of line 3b is shown. In FIG. 1B, an isolated defect 6b is shown, but very close to the left edge of line 3b. And in FIG. 1C, an isolated defect 6c is shown disposed midway between lines 3a and 3b. The defects 6a, 6b, 6c can be commonly evaporated and eliminated when exposed to a laser beam. As a result of the exposure to a laser beam, the left edge of line pattern 3b may develop xe2x80x9ca mouse-nipxe2x80x9d or xe2x80x9ca rat-bitexe2x80x9d as shown in FIG. 2A. Or, although not shown, the left edge of line pattern 3b may develop a peeling. Or, an edge-roughness may be created on the left edge of line 3b as shown in FIG. 2B, which fails to finish in a desired normal line shape. As the feature size of such a photo mask pattern becomes significantly finer and finer, its repairing process at a high accuracy will be very difficult with a laser beam scanning precisely along the edge of the line pattern. Also, since the focusing of the laser beam is limited, the pattern defect repairing at higher accuracy, which may require a laser beam diameter finer than 0.5 xcexcm, will thus be a troublesome task.
Also, mask repairing for removing defects may be carried out by a sputtering method using a focused ion beam (FIB). It is known that when the ion beam is directed to an irradiation area on the mask substrate made of quartz, its gallium (Ga) ions from an ion source are implanted into the quartz substrate, which generates gallium stains and hence decreases the transparency of the substrate. Moreover, the diffused FIB and the beam expansion of the FIB may result in excessive etching around the perimeter of the microscopic defect that needs to be eliminated. The excessive etching generally produces V-shaped grooves around the periphery of the microscopic defect, which are also known as xe2x80x9criverbeds.xe2x80x9d
For solving the above problems, etching a chromium (Cr) film by xe2x80x9ca gas assisted FIB etching processxe2x80x9d is proposed as the pattern defect repairing method for removing the microscopic defect generated on a chromium mask. (K. Aita et al., SPIE, vol. 2512, p. 412 (1995), and J. David Casey, Jr. et al., SPIE, vol. 3096, pp. 322-332 (1997)). Here, the chromium mask has the light-shielding layer of a chromium (Cr) film or a chromium compound film such as chromium oxide (CrOx) film for delineating the required pattern on the quartz substrate. By the gas assisted FIB etching process, the gallium stains or riverbeds are reported to have been eliminated. It is reported that a mask pattern repaired by the above-mentioned pattern defect repairing method could produce an acceptable image level projected with an i-line at a wavelength of 365 nm. In the gas assisted FIB etching process, an etching gas is employed with a high selectivity of etching rates between the mask substrate and the chromium film or the chromium oxide film.
However, it was found that, in finer masks used for exposure by Deep UV (DUV) rays or further shorter wavelength rays, the chromium film repaired by the gas assisted FIB etching process still has noticeable damages to the mask substrate, attributable to the gallium stains or the riverbed.
FIG. 3 is a plan view of a repaired photo mask corresponding to FIG. 1A, which has been repaired by the gas assisted FIB etching process. The chromium mask 1 has an etching burn 5a generated at the aperture 4, very closely disposed to the left edge of line 3b, from removing the microscopic defect 6a with the gas assisted FIB etching process. FIG. 4 is a diagram showing the image intensity profile taken along the line IIIxe2x80x94III of FIG. 3 on a wafer on which an image of the repaired mask pattern is projected. The ordinate in FIG. 4 represents the intensity of the projected image and the abscissa represents locations on the wafer (along a predetermined axis, such as the X axis). In FIG. 4, xe2x80x9cSxe2x80x9d represents the position of the repaired space between lines 3a and 3b, and xe2x80x9cLxe2x80x9d represents the position of line 3b. It is assumed that the exposure conditions in a stepper loading the chromium mask (reticle) are as follows:
As apparent from FIG. 4, the intensity at the etching burn 5a in the aperture of the repaired mask is decreased by more than 20% from that of a non-defect region of the mask. In this way, the unrequired etching burn 5a was unfavorably transferred onto the wafer.
The gas assisted FIB etching process may rarely be effective for repairing microscopic defect on a phase shift mask when it is made of a silicon based material such as molybdenum silicide (MoSi, or MoSix) and used as the film material for producing the light-shielding pattern or the phase shifter pattern. In a step of imaging with the FIB, the phase shifter film may easily be charged up thus interrupting the projection of an image at a higher signal to noise (S/N) ratio and rendering the end point of the etching process hardly detectable with a higher accuracy.
The present invention has been achieved with a view of the foregoing features and its object is to provide a method for repairing a pattern defect, in which the damage against the transparent substrate of a mask is minimized.
Another object of the present invention is to provide a method for repairing a pattern defect, in which the etching of the surface of the transparent substrate of a mask is minimized.
It is still another object of the present invention to provide a method for repairing a pattern defect, suppressing a change in the image intensity through the mask, thereby having a favorable level of the wafer process margin.
It is still another object of the present invention to provide a photo mask having fine pattern and a high transmissivity, having uniform image intensity profiles.
It is still another object of the present invention to provide a method for manufacturing semiconductor devices having miniaturized feature sizes, with a favorable level of the process margin in the lithography process.
A first feature of the present invention involves a method for repairing a defect generated on a mask substrate. The defect may be isolated from normal patterns, or continuous excess patterns protruding from the edge of the normal pattern. More particularly, the method for repairing the pattern defect according to the first feature of the present invention comprises the steps of: (a) determining the irradiation area of an ion beam directed towards a defect by narrowing the irradiation area by a predetermined distance inwardly from the edge of defect; (b) focusing the ion beam onto its irradiation area to remove a part of the defect from its surface so as to leave a thin layer of the defect on the mask substrate; and (c) removing the thin layer by using a laser beam.
According to the first feature of the present invention, a portion of the defect is etched by a FIB process capable of etching locally at a higher accuracy and fineness. Namely, a gas assisted FIB etching process or a FIB sputtering process may be used. While the predetermined distance from the edge of the defect inwardly narrows the irradiation area of the ion beam, the thin layer of the pattern film material is left so that the mask substrate beneath and around the defect is not exposed to the FIB, and then the thin layer is eliminated by exposure to the laser beam. Accordingly, the repairing method according to the first feature of the present invention enables one to perform the repairing process at a higher accuracy and processing facilitation without producing peelings, rat-bites, or edge-roughness. In fact, the pattern film material of the defect next to the edge of the normal pattern is completely eliminated, hence allowing the edge of the normal pattern to be contoured with a higher precision and a better finish. Also, the pattern defect repairing method according to the first feature of the present invention develops the thin layer, hence minimizing the implantation of FIB ions into the transparent mask substrate and the digging of the surface of the mask substrate. The thickness of the thin layer is smaller than that of the light-shielding pattern, allowing the laser beam to give minimum damage to the mask substrate. Damage to the mask substrate may result in deterioration of the transparency of the mask substrate. As damage to the mask substrate is kept to a minimum, a change in the image intensity caused during the pattern defect repairing can be suppressed, hence it is possible to provide a favorable level of the wafer process margin.
The second feature of the present invention lies in a photo mask, repaired by the method explained by the first feature. Namely, the photo mask of the second feature has a mask substrate having a substantially flat surface and pattern delineated on the mask substrate, the pattern has an edge, the edge has a localized specific side wall having an inclination angle differing from that of the remaining sidewall. The localized specific sidewall corresponds to the repaired portion, and can easily be recognized, since the specific sidewall is brighter than other edges in reflective images. The decrease in the image intensity through the mask substrate neighboring the specific sidewall is not higher than 5% compared with that of other portion, since damage to the mask substrate is kept to a minimum.
The third feature of the present invention lies in a method for manufacturing semiconductor device comprising the steps of: (a) generating pattern on a mask substrate so as to fabricate a photo mask; (b) inspecting a pattern defect on the mask substrate; (c) repairing a pattern defect by the method already stated in the first feature; and (d) fabricating a semiconductor device employing the repaired photo mask.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing the invention in practice.